This invention relates generally to improvements in feedthrough terminal pin assemblies and related methods of installation, particularly of the type used in implantable medical devices such as heart pacemakers and the like to decouple undesired interference signals from the device. More specifically, this invention relates to an improved feedthrough terminal pin and capacitor assembly and related installation method, including one or more filter capacitors, and adapted particularly for use in connecting a lead wire or electrode through a hermetically sealed housing to internal electronic components of the medical device while decoupling interference signals against entry into the sealed housing. The invention is particularly designed for use in heart pacemakers (bradycardia), defibrillators (tachycardia), and combined pacemaker defibrillator devices.
Feedthrough terminal pin assemblies are generally known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices especially such as a heart pacemaker or defibrillator or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray or electromagnetic interference (EMI) signals for transmission to the interior of the medical device. In some prior art devices, the medical device has included ceramic chip capacitors which have been added to the internal electronics of the device in an effort to filter and thus control the effects of such interference signals. This internal, so-called "on-board" filtering technique has potentially serious disadvantages due to intrinsic parasitic resonances of the chip capacitors, in combination with permitting EMI entry into the interior of the device housing. In another and normally preferred approach. A filter capacitor has been combined directly with the terminal pin assembly to decouple interference signals to the housing of the medical device.
In a typical construction, a coaxial feedthrough filter capacitor used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin comprises a so-called discoidal capacitor having two sets of electrode plates embedded in spaced relation within an insulative substrate or base, formed typically as a ceramic monolith. One set of the electrode plates is electrically connected at an inner diameter surface of the discoidal structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing or case of the electronic instrument. In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shunting and shielding undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are commonly employed in implantable heart pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a conductive biocompatible metal, such as titanium alloy which is electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entry of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired heart pacing of defibrillating function.
In the past, feedthrough filter capacitors for heart pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor with a terminal pin subassembly which includes the conductive terminal pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See, for example, the terminal pin subassemblies disclosed in U.S. Pat. Nos. 3,920,888, 4,152,540; 4,421,947; and 4,424,5511. The terminal pin subassembly thus defines a small annular space or gap disposed radially between the inner terminal pin and the outer ferrule. A small discoidal capacitor of appropriate size and shape is then installed into this annular space or gap, in conductive relation with the terminal pin and ferrule, by means of soldering, conductive adhesive, etc. The thus-constructed feedthrough capacitor assembly is then mounted within an opening in the pacemaker housing, with the conductive ferrule in electrical and hermetically sealed relation with the housing of the medical device.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively costly and difficult. For example, installation of the discoidal capacitor into the small annular space between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. Moreover, installation of the capacitor at this location inherently limits the capacitor to a small size and thus also limits the capacitance thereof. Similarly, subsequent attachment of the conductive ferrule to the pacemaker housing, typically by welding or brazing processes or the like, can expose the fragile ceramic discoidal capacitor to temperature variations sufficient to create the risk of capacitor cracking and failure.
There exists, therefore, a significant need for improvements in feedthrough filter capacitor assemblies of the type used, for example, in implantable medical devices such as heart pacemakers and the like, wherein the filter capacitor is designed for relatively simplified and economical, yet highly reliable installation with respect to a conductive terminal pin and associated conductive pacemaker housing or shield. In addition, there exists a need for an improved feedthrough assembly having a discoidal capacitor which can be designed to provide a significantly increased capacitance for improved filtering. The present invention fulfills these needs and provides further related advantages.